Since I didn’t find any walkthrough / strategy guide on the Web, I thought I would write my own (using game version 1.03). It’s quite easy to finish the game in a little under 2.5 hours without dying even once. Here’s how:

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The Spirit Engine Speed Walkthrough

Choice of Characters

Peter
Key skills to develop: Sharpshoot, Mana (wait until World 3), Health (wait until World 5). Comments: Sharpshoot is very powerful (the bonus against large creatures is helpful throughout the game).

Zepher
Key skills to develop: Party Heal, Mana (wait until world 3), Health (wait until World 5). Comments: In the game, you need one character that defends. You can do this with a mage (shields) or with a priest (healing). The problem with mages is that the shields get broken down too easily and you end up taking damage anyway. That’s why it’s much better to have a priest.

Matthieu
Key skills to develop: Hand of the Gods (HotG), Mana (wait until world 3), Health (wait until World 5). Comments: HotG rules.

Combat Tips
The character with the most health should always be in front so rotate continuously (use spacebar). Pay close attention to Zepher’s health; he’s the most vulnerable character and also the most important to keep alive. If Zepher ever gets knocked out, switch Matthieu to Party Heal and revive Zepher ASAP. To coordinate your attacks, don’t even try clicking the buttons on the bottom of the screen (it’s almost impossible to get it right). Instead, use 1, 2 and 3 on the keyboard during combat to configure the sequences you want. This allows for perfect control even on the high combat speed setting.

Dangerous Areas
There are only 3 areas (all in World 4) where there’s a (very small) chance of getting killed:
1. The Shifting Sands. You don’t quite have adequate armor yet and some battles can get close.
2. Elan’s Tomb. Same comment as for Shifting Sands.
3. The Sanctum of Dust. A tough battle and I assume it’s possible to die here if you are unlucky.

You will most likely finish the game without dying a single time.

1. Giant’s Cradle

The Prairie Trail
Set Peter’s combat sequence to 3 x Speed Fire (no need to change Zepher’s or Matthieu’s yet). Don’t assign any of Peter’s or Matthieu’s skill points until you reach party level 5.

Homestead
Buy a Blunderbuss, Robes and the excellent Crystal Talisman (+2 mana) for Peter. Talk to Mr. Grimes who tells you about his suitcase.

The Dusty Valley
After the first battle, you’ll find Mr. Grimes’ suitcase (you must have talked to him first). Immediately go return it.

Homestead (again)
Talk to Mr. Grimes who gives you some cash for the suitcase. Use it to buy 2 more robes, as well as a blunderbuss and Crystal Talisman for Matthieu. Assign all of Zepher’s free skill points to Party Heal from now on and use Party Heal as Zepher’s default combat action for the rest of the game (i.e. 3 x Party Heal).

The Dusty Valley (again)
As you win battles and go up in levels, remember to increase Party Heal with Zepher while not assigning any of Peter’s nor Matthieu’s skill points yet.

Hailey’s Glen
After the first combat here, you reach level 5. Assign all of Peter’s free skill points to Sharpshoot from now on and use Sharpshoot as Peter’s default attack for the rest of the game (i.e. 3 x Sharpshoot). Change Matthieu’s attack sequence to 3 x Silver Bullet. Do not assign any of his skill points. The idea is to wait for HotG which is 2 to 3 times more powerful than either Holy Bolt or Silver Bullet. Waiting for HotG is a slight inconvenience in world 1 that pays huge dividends in World 2.

Grundelkrunk’s Lair:
With Zepher Party Healing, Peter Sharpshooting and Matthieu firing Silver Bullets, this battle is won quickly and easily.

Giant’s Toe
(Skip through Hailey’s Glen to get here). Listen to the argument in town. Matthieu can resolve the dispute to receive 2 HotG skill points as a reward (very nice but you’ll have to wait before you can make use of them). Buy the Gnarled Talisman of Party Healing for Zepher (+1 mana as a bonus). Also buy a Leather Jersey for one of your characters. Note: you cannot sell your old armor/weapons. Just trash them.

Icy Ridge
Assign two of Matthieu’s skill points to Party Heal so that he can heal at the rate of 3 (gives you an extra margin of safety). Use Sharpshoot and Silver Bullet if you are fighting against 2 enemies. As soon as you encounter a group of 3 enemies, change your strategy to Party Heal with Matthieu and Sharpshoot with Peter (Zepher keeps on healing). With your two priests healing and Peter sniping, you’ll clear the battles comfortably.

Giant’s Top
Use the same strategy as in Icy Ridge; two healers and Peter Sharpshooting; it’s a long but easy battle.

Total time to finish World 1: 25-30 minutes.

2. Pansylvania

Serinet Bastion
Buy one or two Iron Breast Plates here (depending on how much money you have).

Town GraveYard
Clearing the graveyard isn’t strictly necessary to finishing the game but you get some helpful experience and money by doing so. You should attain level 10 (new skill options) before reaching the Ghost. This is what you have been waiting for. Assign all of Matthieu’s free skill points to HotG from now on and use HotG as Matthieu’s default attack for the rest of the game (i.e. 3 x HotG). Matthieu is now so powerful that rest of the World is a cakewalk. Use Magic Bullet (or Dynamite) with Peter to help in the fight against the ghost.

Torchlit Catacombs
With Peter, always Sharpshoot until all the “solid” monsters are dead, then switch to Dynamite (many opponents) or Magic Bullet (one opponent) to help finish off what remains.

The Misty Marshes
Buy an Emerald Talisman of Blood from the witch. Go out of the area and back in again to buy a second one. Do NOT wear the new talismans yet! The last thing you need for the next area is less mana and more health.

Arachnid’s Grove
As you enter, run to the right and never look back. That “thing” will kill you if it catches up. You need to win the combats quickly so make sure that Matthieu is still wearing that Crystal Talisman. If your health is looking good and you know you can win the fight without further healing, switch Zepher to an attack (HotG) to help speed things up. It’s not really necessary though; Peter’s and Matthieu’s attacks are so powerful that you’ll win these battles in about 10 seconds anyway.

Hamlet
Buy a Boa Hide Coat and an Arquebus for Peter, if you can afford it. Note: the amount of money you get from combat seems to be somewhat random. It’s now time to equip Zepher and Peter with the Talismans of Blood. Give the Gnarled Talisman of Party Healing to Matthieu (he needs +1 mana the most).

Broken Bridge
Ignore the ghosts and focus on Snack (Sharpshoot). When Snack fades into the background, recharge with Peter. If the ghosts are dead and Snack fades into the background, recharge with both Peter and Matthieu and launch your attacks on Snack when he reappears. Overall, it’s a fairly easy battle. Take the key by walking over it.

Hamlet
Go back here to buy an Arquebus for Peter if you couldn’t afford it earlier.

Haunted Church
You need the key from the broken bridge to get in. Put Peter and Matthieu on a Recharge sequence when the blue Bogeymen disappear in the background and blast the beasts when they reappear.

The Defiled Altar
Take a couple Sharpshots with Peter until the kamikaze imps appear. As soon as that happens switch Peter and Zepher to Rifle Attack. They’ll take care of the imps while Matthieu finishes the daemon off. You can try and sneak in a couple Sharpshots between the kamikaze imps but it’s not really necessary. It’s usually over in less than 20 seconds with all your characters at full health.

Eastern Way
No special strategies required here. The bounty hunters won’t even know what hit them.

Total time to finish World 2: 30-35 minutes.

3. Mango Republic

Downtown Heavena
Buy two Copper Plated Jackets. Talk to Djan to reassign your skills. Make sure you assign all your free skills points before doing that otherwise you will lose them (bug in the game).
Peter: +6 Mana, the rest in Sharpshoot
Zepher: +8 Mana, the rest in Party Heal
Matthieu: +8 Mana, the rest in HotG

Note: Mana gives you the initiative at the beginning of combat, enabling you to rapidly knock out at least one enemy. It also allows you to heal your party quickly at the beginning of combat when you get hit the hardest. Overall, this makes you much stronger than using the skill points to further increase Sharpshoot/Party Heal/HotG. You can leave mana at the above mentioned levels until the end of the game.

Republican Government Building
Use Dynamite if the way is blocked by one of those black blobs. Make sure all of it dies (including the small blobs on the floor) before switching back to Sharpshoot. The assassin tends to go for the weakest character with his diving sword attack; just keep rotating your characters. Focus on keeping Zepher alive, and you’ll win easily.

Blue Sapphire Bay
The sea horses have a powerful attack and they are immune to magic (no HotG). Switch Matthieu to Party Heal while Peter takes them out.

Jungle Tracks
Starting from now until the end of the World, Zepher will have lower health (because of the assassin dart). You’ll need to be extra careful that he doesn’t get knocked out in combat. There’s nothing special in this area.

The Hepplekeck Village
Talk to the general to get mission orders.

The Leafy Underbrush
Go to the end and escort the soldier back. The cloud shaped things are immune to Sharpshoot. Use Dynamite to help out or just wait for Matthieu to roast them.

Kaikin Gorge
Kill as many of the tribesmen as you can. The ones that keep regenerating are weak. Hold out until the counter reaches 0 and the bridge is blown up.

The Hepplekeck Village
Talk to the general to let him know that the missions are accomplished. You get a good Carved Oaken Armor.

Gigliana’s Border
You can go have a coffee during this battle. When you come back the plant will be dead and your characters will have higher health than when they started. Ok, perhaps you do have to rotate your characters once in a while but this is one of the easiest battles in the game.

Robert Hat’s Happy Hideout
Buy the Pearl Talisman of Heart (fast healing) and give it to Matthieu (replacing the gnarled one). Increase mana skill by 2 to compensate for the lost mana point as soon as you can. Talk to father Oharne to receive some welcome additional skill points for Zepher.

The Inky Pit
Start Peter with Dynamite. Once the black blob is dead switch Peter to Sharpshoot. Meanwhile rotate Zepher away from the falling knife attacks. It doesn’t matter if the others die in the attack, Zepher will regenerate them. Similarly to the previous battle against the assassin: focus on keeping Zepher alive and you’ll win easily.

Total time to finish World 3: 25-30 minutes.

4. The Mystic Dunes

Saline
Buy a hunting rifle and SpiderSilk Overcoat for Peter. Talk to Kai Kalenzo at the station, then head out of the area and come back in to buy another SpiderSilk Overcoat for Matthieu. You’re now ready to face the desert (leave town by going left).

The Shifting Sands
The battles here are randomly generated and can be fairly tough. The EX NX stone golems are particularly annoying because they have high hit points and can provide cover for beasts behind them. Use magic bullet with Peter to help take them down as soon as possible. The shifters are not too hard. With HotG and Sharpshoot something will always be hitting them hard. If you’re fighting against more than one Dune Wraith, it makes sense to recharge when they fade into the background.

The Magi Retreat
Skip the Aegin Warstick quest, it’s a waste of time. Don’t even come here unless you want to save the game.

Elan’s Tomb
The undead creatures are tough so stay alert during battle. HotG and Sharpshoot complement each other well here (mixture of MX and NX creatures). Get the stone at the end of the passage.
Now walk through Shifting Sands (skip it) and Saline to The Mana Tidebreak.

The Mana Tidebreak
This area has the same creatures as the Shifting Sands (see above)

The Veroonai Enclave
Talk to Tza Tze and fight him. Recharge with Peter when he’s NX and recharge with Matthieu when he’s MX. Or don’t even bother; HotG and Sharpshoot until he cries uncle.

Granael’s Pedestal
Not since Gigliana’s Border in World 3 has any battle been easier. At the end of the fight, the poor beast finally intensifies its feeble attacks but at that point, it’s already all over. Take the stone.
Note: after you kill Granael, beetles will annoy you in all desert battles. They’re not particularly tough but they make Sharpshoot completely ineffective. That’s why you killed Granael last. Head back to Saline through Shifting Sands (skip the battles).

Saline (again)
Buy two Crusader Full Metal Jackets and give one to Peter. Also equip him with the Pearl Talisman of Heart. Walk to the station and take the train.

Stone Circle
After the cutscene you have to fight with Peter alone against two guards. They’re quite tough but with the armor and talisman you should have no difficulties.

Arcane Library
Give the Pearl Talisman back to Matthieu. Use Silver Bullet on the magic-immune Familiars. You can gain 1 extra skill point by reading a book in the library. You can also buy Mist-Wisened Gray Coats here (a good armor that doesn’t slow you down).

The Sanctum of Dust
This is the hardest boss battle in the game but it’s manageable. By now, you should be at just about skill level 30 in Sharpshoot, Party Heal and HotG. Start by taking out the Familiar in front of the mage with Sharpshoot and Silver Bullet. Then switch to HotG and keep firing until the mage’s health goes down to 1000. While he recovers back to health 4000+, you can recover fully too (recharge with Peter and Matthieu; keep healing with Zepher). Round 2 is just the same as round 1. Keep rotating your characters as they fire at the mage and finally kill him.

Total time to finish World 4: 35-40 minutes.

5. Clarke’s Tower

Clarke’s Tower Lobby
When you arrive here you get approx. 5 skill points for each character. Use them to reach 30 in Sharpshoot/Party Heal/HotG if you aren’t at that level already. Any additional points from now on should go to Health (you need good health to survive the laser attacks comfortably.) Talk to Jeeves in the lobby to get the Crafted Golden Rifle for Peter. The laser guys seem tough but as long as your health is 120+, they’re not a problem, even if you’re fighting two of them at the same time. Use Magic Bullet against the Riot Troopers and Sharpshoot for the rest. HotG is good for everything.

Elevator 16
When the gunship moves away, recharge with Peter and Matthieu. With Zepher, use your judgment whether to recharge or keep healing (if your health is low, keep healing). You don’t need to destroy the gunship; you just need to survive for 90 seconds and that’s extremely easy because the gunship can’t keep up with your heal rate.

Prototype Development Wing
Use the same fighting tactics as in the lobby. The beam that comes down from the top is more of an inconvenience than a difficulty. It exchanges health for mana and vice versa. Always rotate Zepher into the beam. Like that he’ll keep shifting between high health / low mana and low health / high mana. Should Zepher get knocked out, you can use the beam to revive him.

Assembly Room
The final battle, and it’s one of the easiest (you are virtually invincible now).
Stage 1: A metal case goes up and down. When it’s up, use Sharpshoot and HotG. When it’s down, recharge with Peter but keep firing with Matthieu.
Stage2: You’re getting fired at with missiles. Make sure that the vertical missiles aren’t hitting Zepher. When horizontal missiles fly at you, temporarily switch Peter to Rifle Attack to blow them up. Then take a few Sharpshots until the next missile comes. Use HotG all the way through.
Stage 3: You can fire at will but you’re also getting hit by some pretty powerful energy bolts. Those will eventually take down your health but you’ll win long before reaching any kind of real danger level.

Note: if you reach a low level of health at any time during the battle (I’m not exactly sure what the trigger level is), Tza Tze comes and puts himself in front of you as a shield. Use that time to heal / recharge or just keep on firing. You’ll most likely win before Tza Tze even needs to intervene.

Total time to finish World 5: 20-25 minutes.

Total time to finish the game: approx. 2.5 hours (combat speed set to high and all cutscenes / dialogues skipped).